We are engaged in a systematic study of the subpopulations of lymphocytes engaged in the humoral and cellular immune response. We have developed a variety of techniques and reagents for the separation of these subpopulations and a number of assays for various lymphocyte functions. A major current interest is in the characterization of the immune response to allogeneic antigens of the major histocompatibility complex, and the study of the non antigen specific factors involved in cell interactions.